The present invention has been developed in collaboration with the Groupement pour la Recherche sur les Echangeurs Thermiques (GRETH), France, for the realization of similation tests.
In-French Patent No. 2,142,624 describes a stackable monolithic refractory element, useful in particular for equipping the regenerator chambers of glass melting furnaces, consisting of at least three radial arms or flanges, of general parallelepipedal shape, integral via one of their edges with a central connecting part, this element being moulded from one or more refractory oxides previously melted and the radiating arms or flanges being identical and having a thickness at least equal to 50 MM.
Such elements comprising four orthogonal radial arms of constant thickness have been and are still marketed by the Applicant. These elements are commonly referred to as "cross-shaped" elements.
In French Addition Patent No. 2,248,748 to the above patent, the Applicant described a variation of embodiment of the element of Patent No. 2,142,624, according to which the radial arms or flanges have a thickness gradually decreasing from their bottom part to their top part, in order to improve the thermal exchanges as a result of convection between the element and the fluid (air) to be heated.
The great advantage of these "cross-shaped" elements is that they facilitate assembly and, with a single type of element, enable the provision of chequerwork having channels of different cross-section through which the gases and the air pass.
During operation of the chequer chambers of a glass melting furnace regenerator, the hot gases or fumes which originate from the furnace during operation enter into the chequerwork via the top part of the regenerator, releasing their calorific energy into the chequerwork, and are then evacuated via a flue. During this time, cold air supplied at the bottom of another chequerwork heated during the preceding cycle recovers the calorific energy and emerges hot at the top of the chequerwork from where it is conveyed to the burners of the melting furnace so as to ensure combustion of the fuel under optimum consumption conditions.
When the fumes pass into the chequerwork, from the top downwards, they not only release their calorific energy but may also, depending on the operation of the furnace and the type of molten glass, deposit dust or elements volatilized in the melting bath.
These deposits have a tendency to obstruct the channels through which the fumes and the air pass. In order to limit this disadvantage, when it occurs, the "cross-shaped" elements used hitherto tend to create hydraulically smooth channels on account of the flat surface of their flanges and owing to very stable assembly inside channels without any overhanging elements.
Moreover, the optimization, i.e. among other things the increase in the thermal efficiency of the chequerwork of the regenerators, is brought about by the increase in the thermal exchanges between the fluid to be heated and the chequerwork of the regenerator. The increase in these exchanges requires intensification of the thermal exchanges during the most limiting phase of the cycle, i.e. the period when the air passes through. In fact, the refractory/air heat exchange coefficient (convection) is several times smaller than that of the fumes/refractory exchanges (radiation). Care must be taken, however, that the means used do not favour excessively the phenomena of deposition on the parts forming the chequerwork and more particularly in the condensation zones.
There therefore exists a need for improved elements intended to equip the chambers of glass melting furnace regenerators which, while minimizing the deposition phenomena when the fumes pass into the chequerwork, improve the thermal exchanges between the fluid to be heated and the chequerwork of the regenerator.
The invention aims to satisfy this need by supplying novel improved ceramic elements for equipping the regenerators of glass melting furnaces.
More particularly, the invention relates to a ceramic element for equipping regenerators of glass melting furnaces, having at least one vertical wall, the mean thickness of which is at the most 40 mm, characterized by the presence, on at least one face of this vertical wall, of a plurality of obstacles forming an integral part of the element, the protrusion of the obstacles relative to the base surface of the wall being at least 5 mm, the ratio of the distance separating two consecutive obstacles in the vertical direction to the said protrusion being comprised between 3 and 15, and the angle x formed by the obstacle with the base surface of the wall in the given direction of movement of the fumes and the angle y formed by the obstacle with the base surface of the wall in the given direction of movement of the air to be heated being such that x is less than or equal to y. The expression "base surface" is understood as meaning the surface which precedes or follows each obstacle.
The novel element of the invention enables a significant improvement in the regeneration efficiency to be obtained. In fact, the surface obstacles present on the parts enable the refractory/air exchanges to be intensified by disturbing the air flows in the vicinity of the walls previously heated by the fumes.
By "mean thickness" of the wall is meant the thickness of a wall with flat faces having no obstacles, which would have the same volume as the wall with obstacles according to the invention. This mean thickness must be less than or equal to 40 mm.
By "protrusion" of the obstacles is meant the maximum height of the obstacle relative to the base surface of the wall. This protrusion must be at least 5 mm and, preferably, at least 10 mm.
The ceramic element may have very different shapes. They may have the shape of elements with radiating arms or flanges such as those described in the aforementioned French Patent No. 2,142,624, in particular the preferred cross shape with 4 orthogonal radiating arms, but they may also have in horizontal cross-section the general shape of a hollow square, of a hollow hexagon, an L shape or quite simply be formed by a single wall.
According to a preferred embodiment, the two faces of the wall are provided with obstacles.
Each obstacle may extend in a continuous manner over the width of the wall or occupy only a fraction of the latter. In this latter case, there will usually be several obstacles arranged side-by-side but spaced from each other. Also, all or some of the obstacles may be only partly present on a given element, their complementary portion being located on the adjacent element arranged above or below in the chequerwork.
The obstacles present on a wall may all be of identical shape, or a mixture of obstacles of different shapes may be used. The distance separating two consecutive obstacles in the vertical direction may be fixed or varied. The important thing is that the ratio of the distance separating two consecutive obstacles in the vertical direction to the protrusion is comprised between 3 and 15, preferably between 5 and 10.
Observance of this ratio range allows the boundary layer of the air, detached from the wall by an obstacle, to rejoin the wall, i.e. the base surface, before reaching the next obstacle. As a result of this, the major part of intensification of this thermal transfer is effected at the rear of the obstacle in the direction of movement of the air and in front of the next obstacle, in the zone where the flow of this air detached from the wall by the obstacle adheres to it again.
According to a preferred embodiment, the obstacles on one vertical wall face are staggered in the vertical direction relative to the obstacles on the other face of this wall. In other words, the wall viewed in vertical section has an asymmetrical profile.
The staggering of the obstacles on the opposite faces of a same wall enables intensification of the thermal transfer to be applied to best advantage. In fact, it results in the positioning of the thickest parts of the wall opposite the zone where the air flow readheres. Thus, the most intense energy transfer is effected at the points on the wall where the calorific capacities are greatest. Staggering of the obstacles offers, moreover, an additional advantage. Assuming the equivalent element to have a mean thickness, an element with staggered obstacles generally possesses a mechanical strength greater than an element with obstacles which are not staggered (symmetrical). This is due to the fact that the element with staggered obstacles has a minimum local thickness greater than that of the part with obstacles which are not staggered.
The obstacles may, themselves, also have a symmetrical or asymmetrical profile. They are preferably asymmetrical, in particular for the zones of the chequerwork where a deposit may occur. The profile of the obstacles may be characterized by the angle x which it forms with the base surface of the wall in the direction of movement of the fumes (usually from the top to the bottom of the element) and the angle y which it forms with this same surface in the direction of movement of the air to be heated (usually from the bottom to the top of the element). According to the invention, x must be less than or equal to y. Preferably, x will be less than y so that the obstacles offer to the flow of the fumes a surface which is hydraulically as smooth as possible in order to reduce the harmful depositions and offer the flow of the air a surface which is hydraulically as rough as possible in order to improve the thermal exchanges and ensure maximum recovery of the heat stored in the ceramic elements during the previous operating cycle.
The elements of the invention may be manufactured by means of moulding in the molten state refractory ceramic compositions usually used for this kind of application, for example compositions based on alumina (for example, a composition which comprises, by weight, 87.5% of alumina , 8% of magnesia and 4.5% of Na.sub.2 O) or based on alumina, zirconia and silica (for example, a composition which comprises, by weight, 50.6% of alumina, 32.5% of zirconia 15.6% of silica and 1.1% of Na.sub.2 O). The composition is melted according to the usual methods commonly used for melting this type of material in an electric arc furnace, a plasma furnace or an induction furnace, and then cast in a mould, for example using the method described in French Patent No. 2,088,185.
The elements of the invention may also be manufactured by moulding of a castable composition such as refractory concrete or a slip, or else by pressing a suitable composition.
The description which will follow, with reference to the drawing, will enable the invention to be properly understood.